The Kingston terrane, southern New Brunswick, Canada: Evidence for an Early Silurian volcanic arc

Abstract

The fault-bounded Kingston terrane of southern New Brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the Kingston Group intruded by mainly granitic plutons and abundant younger mafic sheets. The granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures, indicative of high-level emplacement. Mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the Kingston Group, which is typically steeply dipping and trends northeast to north-northeast. Hence, in contrast to some earlier interpretations, the Kingston terrane does not consist of a bimodal dike complex. The Kingston Group, granitic rocks, and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist–lower amphibolite facies. New U-Pb (zircon) ages of 442 ± 6 Ma from a dacitic tuff unit and 437 ± 10 Ma from a granitic pluton corroborate previous U-Pb dates that had suggested that the volcanic and granitic rocks in the Kingston terrane are Early Silurian, rather than Precambrian, as had been earlier assumed. The felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar U-Pb ages and chemical character. Compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc-alkalic affinity and emplacement in a continental-margin volcanic arc, herein termed the Kingston arc. A U-Pb date of 435 ± 5 Ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault-bounded amphibolite-facies metasedimentary unit. The similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the Kingston terrane, possibly as part of an accretionary complex. If so, its location southeast of the Kingston terrane indicates that subduction was to the present northwest. Mafic sheets in the Kingston terrane are tholeiitic but have some arc-like characteristics, consistent with their origin in a former arc setting, but the sheets were likely emplaced in a subsequent extensional setting. The compositional similarity of the mafic sheets to Late Silurian mafic volcanic rocks and plutons of the Coastal Maine magmatic province suggests that they may be related. A possible model for evolution of the Kingston terrane is proposed, wherein the Kingston Group and related granite formed in an Early Silurian arc on the margin of Ganderia, as represented by the New River terrane of late Precambrian and early Paleozoic age. Northwestward subduction culminated in collision between the Kingston arc and a continental block now represented by the Brookville terrane to the southeast. Subsequent transpressive mo tions in the Late Silurian may have been responsible for the crustal- scale extensional environment in which the Coastal Maine magmatic province was formed, including emplacement of mafic dikes into the deformed former Kingston arc. Transpressive motions—related to juxtaposition of the Avalon and Meguma terranes with the previously accreted, more inboard terranes—continued through the Devonian and into the Carboniferous.